This invention relates to a process for the preparation of asymmetrical N-phenyl-N'-substituted para-phenylene diamines by the reductive alkylation of para-nitroso-diphenylhydroxylamines with an aldehyde or a ketone in the presence of hydrogen and a hydrogenation catalyst.
The reductive alkylation of nitroso compounds by reaction with an aldehyde or a ketone in the presence of hydrogen and an hydrogenation catalyst is known in the art. Thus, use of certain metal compounds as hydrogenation catalysts, such as copper chromite (British Patent No. 804,113 and Russian Patent No. 230,828), nickel sulfide (East German Published Patent Applciation No. 1,542,171), a selenide, a telluride, or a nickel chromium catalyst (Czecholslovakian Patent No. 119,336), or mixtures of two or more heavy metals, iron, manganese, copper, chromium, nickel, silver, cerium or lead, in the form of their oxides, hydroxides, or carbonates (East German Patent Disclosure No. 1,941,009) is known. However, all of the catalysts used in accordance with the foregoing prior art procedures produce side reactions, in particular, reduction of the aldehyde or ketone to the corresponding alcohol.
According to the process for the preparation of N-phenyl-N'-alkyl-para-phenylene diamines disclosed in British Patent No. 1,295,672, para-nitroso-diphenylhydroxylamine is reacted with hydrogen and an aldehyde or a ketone in the presence of a hydrogenation catalyst at temperatures in the range from room temperature to 200.degree. C. The hydrogenation catalyst utilized is made of a metal from Group VIII of the periodic system, such as nickel, cobalt, ruthenium, palladium, or platinum, which, if desired, may have been deposited on an inert carrier material, such as carbon, aluminum oxide, or silica gel. Yields of N-phenyl-N'-alkyl- and N-phenyl-N'-cycloalkyl-para-phenylene diamines of about 95% of theoretical are indeed obtained by the foregoing process, but for an industrial scale preparation of such products, the yield and selectivity achieved thus far are inadequate. Furthermore, the required catalyst quantities are too large and the losses of catalyst are thus considerable.
Therefore, an object of the present invention is to provide a process for the preparation of asymmetrical N-phenyl-N'-substituted para-phenylene diamines by reductive alkylation of para- nitroso-diphenylhydroxylamines with an aldehyde or a ketone in the presence of a hydrogenation catalyst, which process does not suffer from the foregoing drawbacks and results in the production of the desired compounds in high yields.